The invention relates to embossed absorbent paper products, for example, paper towels, tissue and napkins, in which an improved embossing arrangement is used which is particularly suitable for embossing single-ply paper products which have been processed so as to include undulations in the sheet.
Absorbent paper products, such as paper towels, napkins and toilet tissue are widely used on a daily basis for a variety of household needs. These products are commonly produced by depositing cellulosic fibers suspended in water on a moving foraminous support to form a nascent web, removing water from the nascent web, adhering the dewatered web to a heated cylindrical Yankee dryer, and then removing the web from the Yankee with a creping blade which, in conventional processes, imparts crepe bars, ridges or undulations whose axes extend generally transversely across the sheet (the cross-direction). Products produced in this conventional fashion may often be considered lacking in bulk, appearance and softness and so require additional processing after creping, particularly when produced using conventional wet pressing technology. Absorbent sheet produced using the through air drying techniques normally have sufficient bulk but may have an unattractive appearance or undesirable stiffness.
To overcome these deficiencies, an overall pattern is imparted to the web during the forming and drying process by use of a patterned fabric having designs to enhance appearance. Further, through air dried tissues can be deficient in surface smoothness and softness unless strategies such as calendering, embossing, chemical softeners and stratification of low coarseness fibers on the tissue""s outer layers are employed in addition to creping.
Conventional absorbent paper products produced by wet pressing are almost universally subjected to various post-processing treatments after creping to impart softness and bulk. Commonly such tissues are subjected to various combinations of both calendering and embossing to bring the softness and bulk parameters into acceptable ranges for premium quality products. Calendering adversely affects bulk and may raise tensile modulus, which is inversely related to tissue softness. Embossing increases product caliper (bulk) and can reduce modulus, but lowers strength and can have a deleterious effect on surface softness. Accordingly, it can be appreciated that these processes can have adverse effects on strength, appearance, surface smoothness and particularly thickness perception since there is a fundamental conflict between bulk and calendering.
In U.S. Pat. Nos. 5,656,134; 5,685,954; and 5,885,415 to Marinack et al. (hereinafter the Marinack et al. patents), the disclosure of which is incorporated by reference as if fully set forth herein) it was shown that paper products having highly desirable bulk, appearance (including reflectivity) and softness characteristics, can be produced by a process similar to conventional processes, particularly conventional wet pressing, by replacing the conventional creping blade with an undulatory creping blade having a multiplicity of serrulated creping sections presenting differentiated creping and rake angles to the sheet. Further, in addition to imparting desirable initial characteristics directly to the sheet, the process of the Marinack et al. patents produces a sheet which is more capable of withstanding calendering without excessive degradation than a conventional wet pressed tissue web.
Accordingly, using a creping technique it is possible to achieve overall processes which are more forgiving and flexible than conventional existing processes. In particular, the processes of Marinack et al. can be used to provide not only desirable premium products including high softness tissues and towels having surprisingly high strength accompanied by high bulk and absorbency, but also to provide surprising combinations of bulk, strength and absorbency which are desirable for lower grade commercial products. For example, in commercial (away-from-home) toweling, it is usually considered important to put quite a long length of toweling on a relatively small diameter roll. In the past, this has severely restricted the absorbency of these commercial toweling products as absorbency suffered severely from the processing used to produce toweling having limited bulk, or more precisely, the processing used to increase absorbency also increased bulk to a degree which was detrimental to the intended application.
The process and apparatus of the Marinack et al. patents makes it possible to achieve surprisingly high absorbency in a relatively non-bulky towel thus providing an important new benefit to this market segment. Similarly, many webs of the present invention can be calendered more heavily than many conventional webs while still retaining bulk and absorbency, making it possible to provide smoother, and thereby softer feeling, surfaces without unduly increasing tensile modulus or unduly degrading bulk. On the other hand, if the primary goal is to save on the cost of raw materials, the tissue of the present invention can have surprising bulk at a low basis weight without an excessive sacrifice in strength or at low percent crepe while maintaining high caliper. Accordingly, it can be appreciated that the advantages of the present invention can be manipulated to produce novel products having many combinations of properties which previously were impractical.
The objective of the undulatory creping blade of Marinack et al. is to work the web more effectively than previous creping arrangements. That is, the serrulations of the creping blade operate to contact the web rotating off of the dryer in such a way that a part of the web contacts the tops of the serrulations while other parts of the base sheet contact the valleys, thereby forming undulations in the base sheet. This creping operation effectively breaks up the hydrogen and mechanical bonds which link the cellulosic fibers together, thereby producing a smoother, bulkier and more absorbent sheet, which is well suited for consumer use. Creping in accordance with the Marinack et al. patents creates a machine direction oriented shaped sheet which has higher than normal stretch in directions other than the machine direction, that is, particularly high cross-direction stretch.
While the paper products produced with an undulatory creping blade have commercially desirable properties, additional processing in the form of embossing can further add to the properties and appeal of the products. Such embossing can enhance the bulk, softness and appearance of the products. It has been found that the proper selection of emboss element spacing, distribution and orientation can positively impact on the retention or enhancement of the beneficial properties caused by the creping of the web with an undulatory blade. Conversely, improper selection of the emboss element spacing, distribution and orientation can negatively impact, or cause a complete loss of, the beneficial properties caused by the creping of the web with an undulatory blade.
Undulatory blade creping creates a machine direction oriented shaped sheet which has higher than normal stretch in the directions other than the machine direction. The present invention recognizes and takes this three dimensional sheet shape and stretch into consideration. The application of embossing to the biaxially undulatory sheet is done in a way that the emboss process provides the desired modifications to the sheet with controlled extension and disruption of the localized bonds and fiber shapes imparted by the undulatory blade creping. In order to determine the parameters for embossing for sheets processed with an undulatory creping blade certain test embossings were made: when a relatively large size Quilt emboss was applied to undulatory blade creped base sheets made with a number of different blades (tooth spacings being different) unsatisfactory interference patterns are seen. This is a direct result of the relative spacing of the local shape and cross-direction stretch in the sheet to the spacing of the points of application of the force due to the embossing process. At the other extreme, when a very busy and tight spacing of emboss patterns are applied to undulatory blade creped base sheets, most if not all of, the benefits of the undulatory creping is lost.
In accordance with the present invention there were established parameters for embossing webs that have undulations extending longitudinally along a principal undulatory axis and optionally include secondary undulations which extend in the cross (transverse direction) of the web. The parameters must accommodate: the distance at which the undulations are spaced, the total surface area of the design (embossing) elements, the width and length of the embossing elements and the aspect ratio of the elements, and the angular orientation of the embossing elements with respect to the undulations.
It is an object of the present invention to provide processing to provide single-ply paper products that have improved appearance, bulk and strength.
It is another object of the present invention to provide embossing parameters which are compatible with paper webs that have been produced with an undulatory structure.
The embossing parameters of the present invention are applicable to paper webs having undulations running in either the machine or cross-directions regardless of the means used to apply the undulations to the web.